This invention relates to correction of chromatic dispersion and more particularly to a lens/zone plate combination which corrects chromatic dispersion between light of widely differing wavelengths.
Chromatic dispersion results from an optical material having different indices of refraction for different wavelengths of light. Because of this, light of one wavelength passing through the lens will focus to a different point from light of another wavelength also passing through the lens. Chromatic dispersion is a particular problem in CO.sub.2 laser systems with zinc selinide lenses used for applications such as laser welding and laser surgery. Because CO.sub.2 laser light is invisible, visible light from a HeNe laser is routinely used in conjunction with CO.sub.2 lasers as an alignment aid. An example is a laser scalpel employing a HeNe laser to locate the focal point of the CO.sub.2 laser light which performs the surgery.
A major limitation of using a HeNe laser beam for locating the focal point in a zinc selinide CO.sub.2 laser system is chromatic dispersion. The large wavelength difference between light from a CO.sub.2 laser and a HeNe laser, coupled with the dispersion characteristics of zinc selinide, results in the two wavelengths focusing at significantly different locations. The focal points of the two beams must be made to coincide if the HeNe beam is to be useful for locating the focal point of the CO.sub.2 laser.
In conventional lens design, chromatic dispersion is usually corrected by construction of a doublet consisting of two lenses with different dispersion characteristics. A doublet can, in principle, correct for chromatic dispersion between any two wavelengths. A problem arises, however, if one wants to construct a doublet to correct for chromatic dispersion between CO.sub.2 (10.6 .mu.m) and HeNe (0.6328 .mu.m) laser wavelengths. This is the case because zinc selinide and zinc sulfide are the only readily available optical materials that transmit both 10.6 .mu.m and 0.6328 .mu.m radiation. The doublet approach is therefore not practical.
Chromatic dispersion between two wavelengths can also be compensated for with an air spaced doublet consisting of two lenses of the same material. This approach has recently been proposed to correct for the dispersion between 10.6 .mu.m and 0.6328 .mu.m radiation in a zinc selinide system. Major problems inherent with this scheme, however, are that the chromatically compensated focal point is virtual and that it works only for low numerical aperture systems. These problems severely limit the usefulness of this approach in most systems where a real focus and a high numerical aperture are desired.